The invention concerns a method for operating a hydromechanical transmission with a hydrostatic drive comprising a closed hydraulic circuit. At least an adjustable hydraulic pump, a hydraulic fixed displacement motor and an adjustable hydraulic motor are arranged in the closed hydraulic circuit, preferably however two adjustable hydraulic motors. In particular, this invention concerns a method for the continuously variable adjustment of the transmission of a hydromechanical drive in vehicles, construction devices, industrial trucks or lifting devices. These and similar machines with hydrostatic drives will be referred to in the following as work machines, as used in construction or in agriculture and forestry.
The hydromechanical drives relating to the invention designed for work machines have in common that they require a high level of torque at the output shaft of the hydromechanical drive (=high tractive force), for example in order to perform a specific work function such as lifting or transporting loads, while the torque can be lower to perform a simple, fast driving function, for example when moving the work machine. However, the rotational speed of the output shaft of the hydromechanical drive should be high for fast movement or driving so as to allow the work machine to achieve as high a speed as possible. In both cases, possible driving speeds and tractive forces are limited by the power which can be provided by the driving motor, e.g. a diesel engine.
According to the state of the art, two fundamental possibilities are proposed which involve either the arrangement/implementation of intermediate gears or the arrangement of several hydraulic motors which are connected in parallel in the hydrostatic section. In the case of using intermediate gears and only one hydraulic motor, the conversion range for the torque and the rotational speed at constant hydrostatic power must largely be covered by the intermediate transmission. The high speed increase and reduction ratio required in intermediate gears is often complex and elaborate to implement may not always be feasible, depending on the installation space available. What is more, intermediate gears often consume energy to rotate shafts and cog wheels which do not serve to drive the work machine in all operating states but which are inherent to their construction. Intermediate gears are usually heavy and therefore slow in their response to changes in an operating state or in the required torque or rotational speed at the output or input shaft. For example in a two-gear manual transmission, a high level of tractive force is provided by one gear and a high driving speed by the other gear. In most cases, however, the change of gear results in a loss of power and/or comfort, but will at least cause an interruption in tractive power. If a change of gear is to be effected, the vehicle has to be put into a running state in which such a mechanical change of gear is possible. In the case of jaw clutches in particular, the vehicle may even have to be brought to a halt for this purpose. At the least there will be an interruption in tractive power when a mechanical change of gear is carried out.
In the second approach known in the state of the art using a parallel arrangement of hydraulic motors for connection and disconnection, the problem of bulky, heavy and often complex and unwieldy intermediate gears is avoided in that hydraulic force is to be provided flexibly to the hydrostatic drive by means of the connection and disconnection of hydraulic motors. This results in a very large conversion range in terms of the band of rotational speed and torque available to drive the work machine. In addition, the hydromechanical system as a whole can be operated within a better efficiency range when the connection and disconnection for the hydraulic machines does not occur within ranges where the latter demonstrate a low level of efficiency or a high level of power dissipation.
DE 15 55 247 C3 describes a control unit for a continuously variable hydrostatic drive of a motor vehicle, in which one of two adjustable hydraulic motors is deactivated when a maximum rotational speed is reached at the output shaft of the hydromechanical transmission during acceleration, for example of a work machine. The proposal is here is to implement the deactivation of the hydraulic motor at a displacement greater than zero of the hydraulic motor to be deactivated in order to ensure impact-free disengagement. DE 15 55 247 C3 further proposes that the hydraulic motor intended for deactivation purposes comprises a wobble plate/adjustment device that is continuously reduced in its deflection angle as the rotational speed of the output shaft is increased and which is promptly reset to zero when a deflection angle of approximately four degrees is reached, at the same time reducing the delivery volume of the hydraulic pump by the amount exhibited by the displacement flow of the hydraulic motor to be deactivated at a deflection angle of four degrees. The effect of this is that the hydraulic motor to be deactivated does not move into high rotational speed ranges and rotational speed differences do not arise as compared to the output/input shaft, or that the hydraulic motor still in operation undergoes a sudden increase in rotational speed by absorbing the delivery flow of the hydraulic pump since the latter is already/still fully deflected. For the purpose of a further reduction in speed—corresponding to an increase in rotational speed at the output shaft of the hydromechanical transmission—the delivery volume of the hydraulic pump is initially increased once again until the hydraulic pump is fully deflected. After this, a further increase in speed/rotational speed can only be achieved by decreasing the displacement of the hydraulic motor remaining in the drive, for which purpose this hydraulic motor must of course likewise be adjustable.
DE 43 07 616 C2 draws on the method for deactivating or disengaging a hydraulic motor from a hydrostatic drive as described in DE 15 55 247 C3 and proposes in addition that the deactivated hydraulic motor should be braked by means of a mechanical brake. According to DE 43 07 616 C2 this is a measure to prevent the motor from continuing to rotate without load or from exceeding a maximum rotational speed permitted for the hydraulic motor.
For the (renewed) connection or engagement of an adjustable hydraulic motor when reducing driving speed, i.e. when reducing the rotational speed of the output shaft of the mechanical drive, the state of the art proposes that the operational steps for deactivating or disengaging should be performed in the reverse order. In other words, the state of the art proposes connecting or engaging a hydraulic motor at standstill to the hydrostatic drive train that is under load. In this way, the latter is suddenly “pulled into action” by the assigned rotating clutch shaft connected to the mechanical drive (intermediate gear) because the clutch shaft is driven by the other hydraulic motor under load via the intermediate gear. This places a high level of stress not only on the clutch but also on the components of the hydraulic motor to be connected, as well as on the other hydraulic motor remaining in the drive. What is more, the jerking activation of the other hydraulic motor can be felt when operating the work machine.
According to the state of the art, the engaging hydraulic motor is accelerated suddenly from standstill to the rotational speed equal to that of the clutch output shaft, causing an engagement operation which has a sudden impact or is at least of a jerking nature, an effect which can at best be dampened by the use of multi-plate clutches, for example. What is more, the forces of inertia that occur when the hydraulic motor is accelerated are transmitted to the intermediate gear by the clutch forces, and this is felt in the operation of the work machine as a jerk. In order to achieve minimum interventional impact when connecting an additional hydraulic motor to a hydrostatic drive train according to the method proposed by the state of the art, the rotational speed of the assigned clutch shaft must be as low as possible so as to avoid high differences in rotational speed and therefore high forces of inertia between the clutch shaft and the hydraulic motor to be connected.
The object of this invention is therefore to provide a method for the engagement/disengagement of a hydraulic motor to and from a hydrostatic drive train with at least one hydraulic pump and two other hydraulic motors, of which at least one can be connected to or disconnected from a hydromechanical drive train, whereby both the disengagement and engagement operations are impact-free, jerk-free and avoid material stress. At the same time, the method is to be capable of being executed simply and without elaborate technical aids, nor should it incur high costs in terms of technical implementation.
The object is achieved by means of the method according to the invention as described in claim 1, whereby the claims dependent on claim 1 specify other preferred embodiments of the method according to the invention. In the subordinate claim, a control device is claimed with a computer program to execute the method.